Though he wasn’t mainly known as a TV personality per se, he is forever known for Seven Words You Can Never Say on TV (a routine which, ironically, was itself cited in a Supreme Court ruling on what you could and couldn’t say on broadcast TV). Putting his civil and linguistic libertarianism at the center of his work made him a particularly American comedian, one who reminded us—especially during mass quietings like the periods after 9/11 or the Janet Jackson Super Bowl incident—that sometimes the thing you can never say is they thing you most need to.
[Update: Time.com editor, and comedy scholar, Richard Zoglin has an appreciation of Carlin here.]